Many mobile radio systems of various kinds are known and in use. One kind of systems are analogue FDMA systems. Abbreviated names of some well known FDMA systems are AMPS, NMT and TACS.
A type of systems different from analogue FDMA systems are digital FDMA systems. The pan European digital cellular system abbreviated GSM is a type of digital mobile radio communication system now in use in Europe. This system is specified in the document "Recommendation GSM" from ETSI/TC GSM, published by European Telecommunication Standardization Institute, ETSI B.P. 152-F-06561 Valbonne Cedex, France. For an exhaustive information on this system reference is given to the mentioned publication, the subject matter of which is incorporated herein as a reference.
One type of mobile radio communication system used in USA is specified in the document EIA/TIA, Cellular System, Dual-Mode Mobile station--Base Station Compatibility Standard, IS-54, published by ELECTRONIC INDUSTRIES ASSOCIATION, Engineering Department, 2001 Eye Street, N.W. Washington, D.C. 20006, USA. This system has both FDMA radio channels for radio signals with analog modulation and TDMA radio channels for radio signals with digital modulation. For an exhaustive information on this system reference is given to the mentioned publication, the subject matter of which is incorporated herein as a reference.
Both the system according to TIA IS-54 and the GSM system are TDMA systems with many radio channels disposing separate frequency bands. For a bidirectional connection with a mobile, a telephone call, one time slot of a radio channel is required for each direction of the connection. In the older analogue FDMA systems like AMPS, TACS and NMT one entire radio channel is required for each direction of each bidirectional connection with a mobile. An entire radio channel or a time slot of a radio channel, used by a base station for transmitting radio signals including speech or data pertaining to a connection to a mobile station, is sometimes called a forward channel of a connection. Sometimes it is called a downlink of a connection. An entire radio channel or a time slot of a radio channel, used by a mobile station for transmitting radio signals to a base station and including speech or data pertaining to a connection involving the mobile station, is sometimes called a reverse channel of a connection. Sometimes it is called an uplink of a connection. In addition to radio channels for information pertaining to connections already set up, e.g., speech of a telephone call or data of a data connection, most cellular mobile radio systems also have separate control channels for broadcasting system information, setting up calls, paging of mobiles or general information not pertaining to a particular connection already set up.
The radio frequency spectrum available to a mobile radio communication system limits the capacity of the system, the number of simultaneous connections the system can handle. In order to be able to use the same radio channel in FDMA systems, or in TDMA systems the same time slot of a radio channel, for more than one connection, mobile radio systems are made cellular systems. The geographical area to be covered by a system is then divided into smaller areas called cells and mobiles in a cell communicate with a base station for that cell. Cells are grouped together in clusters. Some or all of the available radio channels are distributed among the cells according to a frequency plan. The cell sizes will depend of the required traffic handling capacity. The higher required capacity the smaller cells.
Cell clusters and frequency plans enable plural use of radio channels in a FDMA system and plural use of time slots of radio channels in a FDMA system. Such plural use of radio channels and time slots is sometimes called channel re-use. The interference from other stations using the same radio channel or time slot is sometimes called co-channel interference. The co-channel interference sets an upper limit to the channel re-use. The co-channel interference depends, of course, on the output power of the radio signals transmitted. Thus, transmitting unnecessarily strong radio signals causes unnecessary co-channel interference and unnecessarily limits the capacity of a cellular FDMA or TDMA mobile radio communication system. Thus, appropriate control of transmitter output power is important, at least in high performance cellular FDMA and TDMA mobile radio systems.
There are other reasons for controlling the of power of radio signals transmitted in a cellular system. Power conservation is an important aspect of small light weight portable battery powered mobile stations. One way of saving battery power in a mobile station is to control the strength of transmitted radio signals in response to measured signal strength at the receiving base station. If the signal strength at the receiving base station is not be measured, a mobile must always transmit radio signals with a strength sufficient for a worst case condition, e.g., when the mobile station is located at the borderline of a cell. For most locations such a signal strength is unnecessarily high. If the strength of received signals is measured, a base station may send power control messages to the mobile permitting a reduction of the mobile transmit power whenever an excessive signal level is detected.
Another way of saving power and reducing interference is discontinuous transmission. In a normal telephone call, pauses in the speech are frequent and quite long in relation to a radio channel time slot. Transmitting radio signals when there is no information to forward is only a waste of power. Discontinuous transmission means the transmission is interrupted when there is a pause in the speech of a call or no information to be forwarded on an ongoing connection.
Another type of digital mobile radio communication systems somewhat different from the above described FDMA and TDMA systems is the broadband code division multiple access type systems, abbreviated CDMA. In normal broadband CDMA systems, all the radio signal transmissions relating to different connections involving the mobile stations are not separated in time slots or in different narrow band radio channels. Also in a normal broadband CDMA system there is no fixed frequency plan. Instead base and mobile stations both in the same cell and in surrounding cells may deliberately transmit radio signals relating to various connections simultaneously on the same wideband radio channel. As a consequence, the co-channel interference in a CDMA system will be very high in relation to such interference in the previously described TDMA systems. More precisely the interference level in CDMA systems will normally be several times as high as the level of the desired radio signal relating to the connection.
The reason why a CDMA system can cope with this high level of co-channel interference is the wide bandwidth of each radio channel used. A wideband radio channel in CDMA will normally have a bandwidth equivalent to several of the narrow bandwidth radio channels used in TDMA or FDMA systems. The wide bandwidth allows for a high degree of channel coding. Such coding makes it possible for the mobile and base station receivers to recognize the desired signal from all other signals even though the interference level exceeds the level of the desired signal.
A feature of the CDMA systems is that the number of connections permitted within a frequency band is not limited by the number of time slots/radio channels. Instead the call handling capacity is limited by the maximum level of co-channel interference still permitting the mobile and base station receivers to detect their desired signals.
In a CDMA system, power control and discontinuous transmission reduces the average total power of interfering signals. Thus, discontinuous transmission means reduces co-channel interference and increases capacity in a CDMA system, since the capacity generally depends on the average interference level.
One reason for using CDMA, as opposed to FDMA and TDMA, is that CDMA is alleged to enhance the spectrum efficiency. In all calculations of the spectrum efficiency, i.e., number of connections per cell for a certain bandwidth, all cells have been equally sized.
In a CDMA system, it is very important that the received signal strength from all users on the same wideband channel is equalized. Otherwise an unnecessarily high signal would reduce the capacity since the processing gain of the coding can suppress only a certain amount of interference.
In the reverse channel from mobile to base, the transmit powers of the mobiles should be controlled in order to equalize the received signal strengths at the base station and avoid mobiles close to the base using unnecessarily high powers that would cause unnecessary interference with the signals from the mobiles at the edge of the cell.
In the forward channel from base to mobile, the transmit power distribution over the mobile flock should be tailored according to each mobile's distance from its cell edge. The power of signals transmitted to mobiles close to the cell edge should be increased to compensate for the higher interference level received by that mobile from neighboring base stations.
This technique, known as Dynamic Power Control, is essential for the performance of a CDMA system. The technique works well if all cells are of equal size and all base stations transmit radio signals with the same total output power. A mobile station at the border between two adjacent cells will then receive radio signals of equal power from its own base station and from the neighbor base station. Similarly the two base stations for adjacent cells will receive signals of the same power from a mobile station at the border, and due to the dynamic power control that power will be equal to the power received from the other mobile stations in the cell.
However, a different situation will arise if two adjacent cells are of substantially different sizes. Cells of different sizes may be adjacent at the border between a high traffic area and a low traffic area. It can also occur when a so called "umbrella cell" gives general coverage to an area where smaller "microcells" gives high traffic capacity to certain areas.
In a prior art CDMA system, the output power of the base station in a larger cell would be higher than that of a smaller cell so that a mobile at the cell border would receive signals of equal strength from the two base stations. This would not cause special problems since it is the same situation as for cells of equal size.
However, a mobile at the border between a larger cell and a smaller cell transmitting radio signals to a base station for the larger cell would have to transmit radio signals of higher strength than a mobile at the same cell border transmitting radio signals to a base station for the smaller cell, in order for the strength of signals received at the intended receiving base stations to be the same. The mobile transmitting to the base of a larger cell would thereby cause unacceptable interference to the base of the smaller cell and reduce its capacity.
Alternatively, any mobile station at the cell border could transmit radio signals of the same power, required for the largest cell, independent of the size of the cell the mobile transmits to. The power of signals received at the base station would then be unnecessarily high in small cells. Since the signals received by a base station from all mobile stations in a cell should be equally strong, all mobile stations in the smaller cell would have to increase the output power correspondingly. This would lead to higher power consumption in the mobile stations and higher total interference levels.
Although the problem of power control and co-channel interference when adjacent or neighbor cells have different sizes may be more pertinent to CDMA systems, it is also a problem in FDMA and TDMA systems. In summary, prior art methods of communication and power control may cause problems when a cellular mobile radio system comprises adjacent cells of substantially different sizes. The present invention aims at solving these problems.